Adding Delay in Constraint
You can use a conditional if statement in the Driver expression which waits for a specific frame number before evaluating as true. 

So for example, here a Camera is tracking an object (Sphere-X) based on a Driver but I don't want this to kick in until frame 130 (until then I want the camera to stay at 200 on the x-axis):



Using the expression: 

var if frame&gt;130 else 200


Based on a ternary conditional operator:

valueIfTrue if isConditionTrue else valueIfFalse